This application claims the benefit of co-pending German Patent Application No. 101 04 672.3-12 entitled xe2x80x9cMontageeinheit aus einem Bauteil und mindestens einer Schraubexe2x80x9d, filed on Feb. 2, 2001.
The present invention generally relates to an assembly unit including a component and at least one screw. More particularly, the present invention relates to an assembly unit including a bush being made of plastic material.
Generally, assembly technology increasingly requires assembly units in which a plurality of components forms a pre-assembled unit. The assembly unit including a component and usually a plurality of screws is then connected to a work piece. As a typical example for this technology, an upper element and a lower element have to be connected by one or more screws. The upper element as the component and the plurality of screws are combined to form a pre-assembled assembly unit to which the screws are captively connected. The assembly unit is then connected to the lower element by tightening the screws one after the other or simultaneously, especially with the use of a multi-spindle automatic screwing apparatus. Such assembly units are typically used in the production of automobiles. For example, the cast cover of a cast housingxe2x80x94which for example is made of plastic, aluminum or magnesiumxe2x80x94or a cover of a transmission forms the component which is to be connected with the work piece by screws. In this case, the work piece means the housing and the housing of the transmission, respectively. Realizing such assembly units allows for a great level of pre-manufacture. The screws are already located at the component in a captive, aligned and pre-assembled way. During transportation, they are safe from being damaged, and they allow for fast connection with the work piece.
An assembly unit is known from International Application WO 95/21335. The known assembly unit includes a component, for example a cover of a housing, and at least one fixing element in the form of a screw. A third element is associated with each screw. This third element is a bush which is used to capture the screw. The bush is made of metal, especially of sheet metal. The bush increases the cost of manufacture of the known assembly unit. The bush is pressed into a respective through hole being located at the component which requires a separate pre-assembly step. The bush is fixed in the through hole of the component due to frictional engagement. The bush has a structural height which is a multiple of the structural height of the component in the region of the through hole. The screw itself also has a respective complicated shape and a great axial length. A ring flange is located in the region of the shank of the screw, the ring flange including ring channels at both sides. The ring flange has to be produced at the shank of the screw in addition to the thread. The size of the ring flange is coordinated with the size of a collar being located at the bush in a way that an axial lock is formed, the axial lock still allowing for a respectively great axial movement in a certain region of movement in a way that the screws in the pre-assembled position are located to be captive, but not fixed at the component. There also is the possibility of not arranging a ring flange at the screw. In this case, special elements have to be arranged at the bush, the elements in combination with the screw forming an axial lock. Consequently, the bush gets a more complicated design. In this case, the thread of the screw may fulfill part of the axial lock. There only is a relation between the diameter of the through hole being located in the component and the diameter of the shank of the screw in a way that the screw together with the bush pass through the through hole and have to be inserted into the through hole, respectively. However, exact axial alignment of the screw at the component does not take place since the screw is arranged to be only movable to a limited extent. This fact may be advantageous for screwing the screw into the work piece if it is ensured that the screw finds the entrance into the thread being located into the work piece in a centered way. Due to the possibility of tilting the axes of the screws with respect to one another, entrance of the free end of the shank into the thread in the work piece may only be realized to some limited extent. These disadvantages may be counteracted by the bush as third component within the assembly unit having a relatively great axial length. Especially, it may be designed to be substantially longer than the thickness of the wall of the component. Due to the length of the bush, the length of the screw also has to be increased. As a result, not only costs are increased, but in some cases there is a negative effect on function. Using such bushes in assembly units including at least three single elements is disadvantageous since the bush is subjected to tension while the screw is being screwed into the work piece. In case the bushes are made of comparatively thin deformable sheet metal, there even is the danger of different deformations of the bushes occurring during tightening of the screws in a way that it is only possible to apply reproducible pre-tensioning forces to a limited extent.
German Patent Application No. 196 51 838 A1 shows a component being made of plastic material, the component being designed to be connected to a work piece by screws with an intermediate arrangement of a seal. A bush being made of a metallic material, protruding through the component and being connected to the seal is used. The screw has to include a separately produced ring flange which cooperates with the bush in the sense of an axial lock. In this way, the screw is arranged in a lose, but captive way at the bush. The bush is a third element of the assembly unit in addition to the component and to the screw. The bush does not necessarily have to extend through the entire through hole in the component.
German Patent No. 195 46 072 C2 shows a bush having a shorter axial length and including a conical collar only partly engaging a partially conical through hole in the component under frictional engagement. The captive arrangement is realized by frictional forces in a way that the construction is respectively unsafe. The bush being designed in the form of a disc finally is located between the supporting surface at the head of the screw and the respective counter surface at the component. In this way, the bush is clamped, and there generally are similar disadvantages, as this has been described above.
The present invention generally relates to an assembly unit. More particularly, the present invention relates to an assembly unit including at least one component and at least one screw including a shank having an outer diameter and at least partially including a thread. The head of the screw is designed and arranged to rotate the screw. A supporting surface of the screw faces the component. The component for each screw includes a through hole having a diameter. At least one bush is associated with the at least one screw, and it is designed and arranged to be insertable into the respective through hole. The bush is made of plastic material, and it is designed and arranged to be fixedly connected in the through hole resulting in elastic deformation of the bush. The inner diameter of the bush is slightly more than the outer diameter of the shank. The supporting surface is designed and arranged to operatively contact the component after assembly without contacting the bush to transmit an axial force. The bush, the screw and the component are designed and arranged to be captively connected.
The novel assembly unit is also suitable for screws which do not produce a thread, for example screws having a metric thread, and in which the respective bush of the pre-assembled assembly unit is not located in the force flux while the screw is screwed into the work piece. Furthermore, the screw and the screws, respectively, are captively connected to the component in the pre-assembled position. To be captively connected is to be understood as being connected in a way that the screw is detachable from the component, but it cannot get lost unintentionally.
The novel assembly unit includes a bush being made of plastic material instead of metallic material. The bush may be produced in a simple fashion as a preform element. The bush is connected in the through hole of the component under elastic deformation with frictional engagement and/or with form fit. Frictional engagement is also to be understood as force connection which, for example, is effected by a micro-encapsulated glue applied onto the outer surface area of the bush or onto the cylindrical surface of the through hole in the component. The through hole may be designed as a simple bore. The inner diameter of the bush is only slightly greater than the outer diameter of the thread of the screw. The screw only contacts the bush in the region of a collar protruding towards the inside. The screw is centrically guided by the length of the bush. Assembly of the screw and of the bush advantageously acquires low assembly forces. Pre-assembly may be well automatized since simple pressing of the screw with respect to the bush is only necessary without any turning movement or locking movement. The screw may be pressed into the bush being located in the component. However, it is also possible to first push the bush onto the screw and to then introduce the bush into the component. Due to the use of the bush being made of plastic material, there is no danger of contact corrosion, as it occurs between a bush being made of steel and a component being made of magnesium.
The novel assembly unit includes one or more screws which may also have a metric thread. However, it is also possible to use screws which form or produce a thread, preferably screws which produce a thread during screwing without producing chips. There is a certain relation between the diameter of the through hole being located in the component, the bush which is to be arranged in the through hole and the size of the screw, especially in the region of its shank including a thread. In this way, the screw extends through and engages the through hole, respectively, after pre-assembly. Thus, not only the axial lock between the collar and the thread is realized, but also alignment of the axis of the screw and of the component, the alignment being sufficient for centering and resulting from the coordination of the inner diameter of the bush with respect to the outer diameter of the thread in the section of the shank. The alignment of the axis of the screw is determined in this way, and the screw may easily find the hole, especially the threaded hole, in the work piece. In case the screw only partly protrudes into the bush being located in the through hole, the thread of the screw is partly located in the region of the bush and partly at the side of the screw facing the head outside the through hole. In this way, the thread is located in a safe position during transportation from the manufacturer of the pre-assembled assembly unit to the place where the assembly unit is further used such that damages are not to be expected. The screw or the screws are located with aligned axes in the respective bushes (and these in the through holes). As a result, function is improved and a captive arrangement is realized.
Usually, the supporting surface being located at the screw and serving to transmit the axial force is designed to directly contact the component without contacting the bush. Especially, the application and effect of the axial force is not negatively influenced in the tightened position of the screw. The bush is made of plastic material, and it is not loaded by the thread of the screw when the screw is screwed into the work piece. The bush in the tightened position of the screw remains free from any axial force such that negative setting effects in combination with decreasing pre-tension are not to be expected. However, to attain a sealing effect between the work piece, the component and the head of the screw, it may make sense to size the bush in a way that a limited axial force being necessary to attain the sealing effect is transmitted by the bush. However, these axial forces are comparatively low such that no setting effects are to be expected in such a case.
It is also possible to chose a design in which the supporting surface being located at the screw indirectly contacts the component to transmit the axial force with or without using contact with the bush. In this case, a fourth element per screw is added to the novel assembly unit. This fourth element is a supporting disc at one side being supported at the component and at its other side being supported at the supporting surface being located at the head of the screw. The supporting disc may have Z-like cross section. This design is especially advantageous in case great supporting surfaces between the component and the supporting disc are required to maintain defined surface pressure. The use of a supporting disc allows for realizing comparatively great diameters of the supporting surface on the component, these great diameters in case of an integrated design, for example in case of a collar screw, being hard to be produced and making no economical sense. In case of using the supporting disc, there is the further advantage of a comparatively small head frictional moment occurring during tightening of the screw. On the other hand, the supporting disc may also be designed as one piece with the screw, meaning it may be integrated into the head of the screw.
It may make sense that the screw in the region of its shank close to the supporting surface includes a ring groove which forms an undercut. The ring groove has a diameter which is less than the inner diameter of the bush and an effective axial length which is more than the length of the bush. Especially when a plurality of such screws is to be located in the pre-assembled position of the component in a way that it is designed for multiple screw connections with the work piece, it is of importance that the screw in the region of the shank between the supporting surface at the head and the threaded portion includes a ring groove forming an undercut. The diameter of the ring groove is less than the inner diameter of the bush, and its axial length is more than the axial length of the through hole in the component taking the respective transition radiuses into account. Due to the arrangement of the screw, the axial lock and the alignment of the axis as well as the captive arrangement of the mounted assembly unit are reached in the pre-assembled unit. On the other hand, the screw and the screws, respectively, in the region of the ring channel forming the undercut gets free from the bush being located in the component in a radial and in an axial direction such that there is an adaptation of the screw or the screws at the work piece taking
When a plurality of screws is used, a possible clamping effect of the component at the work piece is also prevented. Consequently, the component reaches a free relative position with respect to the work piece. This adaptation may also be used when the screws are screwed into the work piece one after the other. The same applies when a plurality of screws or all screws are screwed into the work piece simultaneously. In this case, it is especially advantageous to use screws which produce a thread, and not to arrange threads in the bores in the work piece which are engaged by the screws. Instead, the bores may be designed as cast bores or cut bores. Then, it is not necessary to produce threads in the work piece during its manufacture. Instead, the screw produces the thread. When respective tolerances are met, it is also possible to connect the pre-assembled assembly unit including the component and the screws with a work piece the bores of which are associated with the screws and already include threads. This applies no matter whether a plurality of screws is simultaneously screwed into the work piece or one after the other. The use of bushes being made of resilient plastic material is advantageous.
The bush at its end portion facing the head of the screw may include a collar protruding towards the outside, the collar forming a stop during insertion of the bush into the through hole. In this way, the bush is prevented from being pressed too deeply into the component and into the work piece. In combination with this arrangement, the screw includes a continuous depression being designed as an avoiding channel and being located close to the supporting surface to prevent clamping of the material of the bush during tightening of the screw. The depression is designed and arranged to accept the collar protruding towards the outside. Usually, the height of the depression or deepening is more than the height of the collar of the bush. In such a case, the bush remains totally free from an axial force of the screw. In case the height of the deepening is less than the height of the collar of the bush, part of the axial force is consumed by clamping the collar of the bush. Even in this case, the essential and greater part of the bush remains free from the effect of an axial force.
The bush may be designed to be fixedly arranged in the through hole of the component due to frictional engagement and/or due to form fit. In this way, the bush is securely held in the through hole of the component, especially in a way that it cannot be rotated. Fixing the bush in the through hole of the component due to frictional engagement and/or due to form fit may be achieved by the application of glue onto the outer surface of the bush. Advantageously, micro-encapsulated glue is used for this purpose, the gluing effect of such glue starting when the bush is pressed into the through hole of the component. The through hole in the component and/or the outer diameter of the bush may also have an unround design. For example, it may be designed as a polygon including a radial protrusion, to have an oval cross section, as a square or the like. The fixed connection due to frictional engagement is realized by a respective size and by respective pressing the bush into the through hole. This may be achieved in different ways. The bush may be arranged in the through hole with an excess or with a surplus excess at both sides, at one side or without an excess.
The bush may have a greater structural height than the component in the region of the through hole. The bush at its side facing the head of the screw may include a collar protruding beyond the through hole in a radial direction or respective protrusions which limit the capability of pressing the bush into the through hole of the component. The collar and the protrusions form a stop when the screw is screwed in, and they prevent the bush from being pushed out off the through hole. Even without a collar and protrusions, the bush at least at its side facing the head of the screw outside the height of the component in the region of the through hole may be designed to be expandable by screwing in the screw. Consequently, the excess of the bush extending in an outward direction forms a stop protruding in a radial direction when the screw is screwed in. The bush may include a plurality of protrusions being designed to be elastically deformed, the protrusions being arranged to face towards the inside. The inwardly protruding protrusions of the bush may include a chamfer to simplify insertion of the screw.
The thread being located at the shank of the screw may have an axial length which is less than the thickness of the component. In this way, there is the possibility of the screw being securely located within the bush during transport of the pre-assembled unit such that there is no danger of damages.
The screw at the free end of its shank may include a centering portion without a thread, the centering portion having an outer diameter being less than the inner diameter of the bush. The centering portion is advantageous when the screw is inserted into the bush and when the screw is screwed into the threaded hole or the thread in the work piece.
The deepening next to the supporting surface may have an axial length which is less than the axial length of the collar of the screw such that a sealing effect is attained in the tightened position of the assembly unit. Only a small part of the axial force of the screw is necessary for pressing the collar of the bush in an axial direction to attain the sealing effect. The greater part of the bush also remains free from forces.